# std::jthread::jthread

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< cpp‎ | thread‎ | jthread

jthread() noexcept; | (1) | (since C++20)
jthread( jthread&& other ) noexcept; | (2) | (since C++20)
template< class F, class... Args > explicit jthread( F&& f, Args&&... args ); | (3) | (since C++20)
jthread( const jthread& ) = delete; | (4) | (since C++20)

Constructs new `std::jthread` object.

1) Creates new `std::jthread` object which does not represent a thread.
2) Move constructor. Constructs the `std::jthread` object to represent the thread of execution that was represented by other. After this call other no longer represents a thread of execution.
3) Creates new `std::jthread` object and associates it with a thread of execution.

The new thread of execution starts executing:

std::invoke(`decay-copy`(std::forward<F>(f)), get_stop_token(), `` `decay-copy`(std::forward<Args>(args))...) | (until C++23)
std::invoke(auto(std::forward<F>(f)), get_stop_token(), auto(std::forward<Args>(args))...) | (since C++23)

if the expression above is well-formed, otherwise starts executing:

std::invoke(`decay-copy`(std::forward<F>(f)), `` `decay-copy`(std::forward<Args>(args))...). | (until C++23)
std::invoke(auto(std::forward<F>(f)), auto(std::forward<Args>(args))...). | (since C++23)

The calls of `decay-copy` are evaluated(until C++23)The values produced by auto are materialized(since C++23) in the current thread, so that any exceptions thrown during evaluation and copying/moving of the arguments are thrown in the current thread, without starting the new thread.
These overloads participate in overload resolution only if std::remove_cvref_t<F> is not the same type as `std::jthread`.
If any of the following is false, the program is ill-formed:

- std::is_constructible_v<std::decay_t<F>, F>
- (std::is_constructible_v<std::decay_t<Args>, Args> && ...)
- std::is_invocable_v<std::decay_t<F>, std::decay_t<Args>...> ||
  std::is_invocable_v<std::decay_t<F>, std::stop_token, std::decay_t<Args>...>

The completion of the invocation of the constructor synchronizes with the beginning of the invocation of the copy of f on the new thread of execution.
4) The copy constructor is deleted; threads are not copyable. No two `std::jthread` objects may represent the same thread of execution.

### Parameters

other | - | another `std::jthread` object to construct this `std::jthread` object with
f | - | Callable object to execute in the new thread
args | - | arguments to pass to the new function

### Postconditions

1) `get_id()` equal to `std::jthread::id()` (i.e. `joinable()` returns false) and get_stop_source().stop_possible() is false.
2) other.get_id() equal to `std::jthread::id()` and `get_id()` returns the value of other.get_id() prior to the start of construction.
3) `get_id()` not equal to `std::jthread::id()` (i.e. `joinable()` returns true), and get_stop_source().stop_possible() is true.

### Exceptions

3) std::system_error if the thread could not be started. The exception may represent the error condition `std::errc::resource_unavailable_try_again` or another implementation-specific error condition.

### Notes

The arguments to the thread function are moved or copied by value. If a reference argument needs to be passed to the thread function, it has to be wrapped (e.g. with std::ref or std::cref).

Any return value from the function is ignored. If the function throws an exception, std::terminate is called. In order to pass return values or exceptions back to the calling thread, std::promise or std::async may be used.

### Example

Run this code

    #include <chrono>
    #include <iostream>
    #include <thread>
    #include <utility>

    using namespace std::literals;

    void f1(int n)
    {
        for (int i = 0; i < 5; ++i)
        {
            std::cout << "Thread 1 executing\n";
            ++n;
            std::this_thread::sleep_for(10ms);
        }
    }

    void f2(int& n)
    {
        for (int i = 0; i < 5; ++i)
        {
            std::cout << "Thread 2 executing\n";
            ++n;
            std::this_thread::sleep_for(10ms);
        }
    }

    class foo
    {
    public:
        void bar()
        {
            for (int i = 0; i < 5; ++i)
            {
                std::cout << "Thread 3 executing\n";
                ++n;
                std::this_thread::sleep_for(10ms);
            }
        }
        int n = 0;
    };

    class baz
    {
    public:
        void operator()()
        {
            for (int i = 0; i < 5; ++i)
            {
                std::cout << "Thread 4 executing\n";
                ++n;
                std::this_thread::sleep_for(10ms);
            }
        }
        int n = 0;
    };

    int main()
    {
        int n = 0;
        foo f;
        baz b;
        std::jthread t0; // t0 is not a thread
        std::jthread t1(f1, n + 1); // pass by value
        std::jthread t2a(f2, std::ref(n)); // pass by reference
        std::jthread t2b(std::move(t2a)); // t2b is now running f2(). t2a is no longer a thread
        std::jthread t3(&foo::bar, &f); // t3 runs foo::bar() on object f
        std::jthread t4(b); // t4 runs baz::operator() on a copy of object b
        t1.join();
        t2b.join();
        t3.join();
        std::cout << "Final value of n is " << n << '\n';
        std::cout << "Final value of f.n (foo::n) is " << f.n << '\n';
        std::cout << "Final value of b.n (baz::n) is " << b.n << '\n';
        // t4 joins on destruction
    }

Possible output:

    Thread 2 executing
    Thread 1 executing
    Thread 4 executing
    Thread 3 executing
    Thread 3 executing
    Thread 4 executing
    Thread 2 executing
    Thread 1 executing
    Thread 3 executing
    Thread 1 executing
    Thread 4 executing
    Thread 2 executing
    Thread 3 executing
    Thread 1 executing
    Thread 4 executing
    Thread 2 executing
    Thread 3 executing
    Thread 1 executing
    Thread 4 executing
    Thread 2 executing
    Final value of n is 5
    Final value of f.n (foo::n) is 5
    Final value of b.n (baz::n) is 0

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3476 | C++20 | overload (3) directly required (the decayed types of) `F` and the argument types to be move constructible | removed these requirements[1]

1. ↑ The move-constructibility is already indirectly required by std::is_constructible_v.

### See also

(constructor) | constructs new `thread` object (public member function of `std::thread`)
C documentation for thrd_create

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